This invention relates generally to the field of clutch assemblies for engaging and disengaging a source of driving power to a moveable element, and more particularly to that type of clutch assembly generally known as the wrap spring clutch. Clutch assemblies of one type or another have long been well known, since they constitute a very convenient and relatively simple mechanism for alternately connecting and disconnecting a movable element either from a source of driving power for moving the element, or from a fixed member for arresting already occurring movement of the element. Possibly the most common and well known form of clutch assembly is that in which a pair of friction members are caused to engage and disengage each other by an external force. One of the friction members is connected either to the source of driving power for moving the movable member, or to the fixed member for arresting movement of the movable member in the manner of a brake, depending on the function of the clutch, the other friction member being connected to the movable element. When the friction members are disengaged, the movable element is non-driven or non-arrested, as the case may be; when the friction members are engaged, the movable element is connected either to the source of power or the fixed member, as the case may be, for moving the movable element or for holding it stationary.
One common form of clutch assembly is the so called wrap spring clutch, which is utilized most frequently, if not exclusively, for controlling the rotary motion of an element mounted for rotation about a central axis, such as a shaft. In this environment, the clutch member which connects and disconnects a source of driving power from a driven element is simply a coil spring having a pair of outwardly projecting tabs or tangs, one on each end of the coil, for connecting the coil to either the source of power or the fixed member, as the case may be, and the driven element. The wrap spring clutch has considerable utility in that it can be used both for driving an element mounted for axial rotation or for stopping the rotary movement of an element already in motion.
There are several major advantages of the wrap spring clutch which render it highly suitable for the foregoing purpose. One is that it requires very little input motion from an external source to change the operating mode of the clutch from engaged to disengaged and vice versa. Another is that either change in operating mode occurs with very little motion of the clutch spring, thereby permitting the change to be virtually instantaneous. A still further advantage is that it exerts tremendous gripping force on the movable element when it is engaged therewith. The combined effect of these advantages is that the wrap spring clutch is very convenient and effective in situations where it is necessary or desirable to achieve extremely rapid acceleration or deceleration, as the case may be, of a movable element.
A problem that often develops in the use of wrap spring clutches, particularly in the situation where it is used to arrest the movement of a rotating element, is that the extremely rapid deceleration of the element generates a shock load in the wrap spring which places sufficient stress on the juncture of the fixed tang with the first coil of the wrap spring that the tang breaks off the coil due to fracture of the metal at the juncture. When this occurs, the spring, of course, is useless and must be replaced. This results in both unproductive down time of the equipment in which the wrap spring clutch is being utilized and the expense of the labor required to disassemble various parts to gain access to the wrap spring for replacement and to reassemble these parts. This was the major problem with the apparatus described in the above mentioned patent application.
Attempts to solve this problem by the use of heavier, stronger springs have not proved satisfactory since the resistance of the spring to movement in the unwinding direction to release the movable element for motion often exceeds the force that is available to move the tang on the movable end of the spring. This is particularly so in the case of the machine in which the wrap spring clutch of the present invention is utilized, since, as will be seen in detail below, the force for moving the spring in the unwinding direction is provided only by a spring loaded device which controls slack in a strip of paper tape.